Activated carbon is a microcrystalline, nongraphitic form of carbon which has been processed to increase its porosity. Activated carbon is typically characterized by a large specific surface area. This large surface area enables activated carbons to act as a very effective absorbent as a result of the high degree of surface reactivity. Favorable pore size makes this surface area accessible to gases and liquids. Generally, the larger the surface area of the activated carbon, the greater is its adsorption capacity. Activated carbons are used in processes to efficiently remove pollutants from liquid and gaseous streams.
Different kinds of raw materials have been made into activated carbons, including plant material, peat, lignite, soft and hard coals, tars and pitches, asphalt, petroleum residues and carbon black. Coal has been found to be a good raw material for the production of activated carbons.
The preparation of activated carbons generally involves two steps. During the first step, noncarbon elements are eliminated as volatile gases by pyrolytic decomposition of the starting material. Where the feed stock contains water, the first step results in the production of steam. Once ‘dry’, a portion of the carbon feed stock is removed through devolatilization. As much of the volatile portions of the feed stock as possible is removed with the goal of only fixed carbon (FC) remaining along with an unavoidable residue of ash. The ‘pores’ of the remaining carbon, i.e. the FC, have been exposed by the devolatilization of the feed stock.
The second step involves a gasification reaction occurring at high temperature. During this step, the diameter of the pores is enlarged, thus increasing the volume of the pores. Typical reactions taking place in the furnace include the following:C+H2O→CO+H2 C+CO2→2COO2+H2→2H2OO2+2CO→2CO2 CO+H2OCO2+H2 The H2O is introduced into the reaction in the form of steam, the C is primarily the FC resulting from the first step and the remaining reactants are free gaseous molecules.
Gasification converts the carbonized raw material into a form that contains the greatest possible number of randomly distributed pores of various shapes and sizes, and a final product with a high surface area.
Besides the activated carbon, outputs of the two steps described above include steam and volatile matter, both from the first step. It is known that steam may be brought from an area of a reaction where it is in excess to an area where it is required. U.S. Pat. No. 4,455,282 to Gerald Marquess and David J. Nell brought waste steam from a drying step into the oxidation step, where it was needed for the oxidation reactions.